1. Field of the Invention
The present invention relates to a temperature control apparatus. In particular, the present invention relates to a microcomputer controlled digital thermostat.
2. Description of the Prior Art
With the dramatic increase in energy costs over the past decade, and the growing concern over the availability of fuels, increased attention has been focused upon energy conservation. The reduction of energy consumption in home heating and air-conditioning has been a topic of widespread interest.
Significant energy saving in home heating and air-conditioning can be achieved by the use of temperature setback at one or more periods during each day. For example, by decreasing the control temperature setting of a thermostat during sleeping hours and during times of day when no one is at home, significant savings in home heating costs can be achieved. Similar savings can be achieved by changes in the control temperature setting of a thermostat during times when air-conditioning is desired.
Clock thermostats which provide at least one setback period have been available for many years. Originally, these clock thermostats were of a mechanical or electromechanical type.
With the development of microprocessors and microcomputers, computerized thermostats which automatically provide two or more setback and setup temperature periods have been developed. In general, these computerized thermostats have included a user interface though which the user can enter selected time and temperature information. This information is stored by the microcomputer and used in controlling operation of a furnace and in some cases an air-conditioning compressor. In many cases, computerized thermostats allow the user to store multiple time and temperature settings for each day of the week.
U.S. Pat. Nos. 4,274,145 and 4,316,256, which are assigned to the same assignee as the present application, describe a highly advantageous electronic digital thermostat which utilizes a microcomputer control circuit. The thermostat described in these two patents controls operation of heating and air-conditioning as a function of sensed temperature and the stored time and temperature data which is entered by the user through a keyboard. The time and temperature data is stored within a read/write random access memory (RAM) within the microcomputer.
Microcomputer based electronic thermostats have, in the past, required a backup power supply, in the form of a battery, even when the primary source of power for the microcomputer and other electronic circuitry is derived from AC line power present in the furnace circuit. A backup power supply is necessary because the AC line power is subject to power interruptions and "brown out" conditions. In either case, a low voltage condition (caused by either a prolonged power failure or brown out which drains the battery) causes a loss of the time and temperature data stored in the read/write RAM. This means that when power is restored after a prolonged interruption, the user must re-enter all of the time and temperature data which has been lost due to the low voltage condition. Depending upon the digital thermostat and the desires of the user, it may be necessary to reprogram over one hundred time and temperature settings in the event of a power outage.
In general, digital thermostats having a battery backup power supply include a circuit which allows current to be drawn from the battery only in the event of a loss of AC power. This circuit prevents current from being drawn from the battery when there is a source of AC power available. This circuit, however, normally exhibits a time delay before the battery is connected into the circuit. This time delay basically represents the turn-on time of the transistors in the circuit. While the delay time is relatively short, the microcomputer can in some cases already be exposed to a low supply voltage condition before the battery is connected into the power supply circuit. This exposure to low supply voltage causes the microcomputer to "lock up" to an inactive condition.
Another disadvantage of a battery backup power supply in a digital thermostat is that the battery periodically requires replacement if power interruptions have occurred. The user may overlook the need for replacement of the battery until it is too late, and the time and temperature settings stored by the digital thermostat have been lost due to a power interruption. In addition, the replacement of batteries represent a continuing expense to the user.
There is a continued need for improved digital computerized thermostats which continue to operate reliably after a power interruption, and which avoid the inconvenience and expense of prior art backup power supply circuits.